The goal of this project is to evaluate the feasibility of using Scanning Unnatural Protease Resistant (SUPR) peptides as molecularly targeted radiotherapeutics for treatment of cancer. We propose that SUPR peptides are optimal molecules for specifically delivering radionuclides to tumors since SUPR peptides possess features that are ideal for systemic targeted radionuclide therapy: SUPR peptides bind with antibody affinities and specificities that lead to high levels of tumor accumulation, high tumor retention, and selective tumor uptake, yet the small size of SUPR peptides leads to better tumor penetration and higher rates of clearance from the body as compared with antibodies. By increasing tumor uptake, SUPR peptides can increase the fraction of radionuclide delivered specifically to the tumor, and by increasing clearance, SUPR peptides can decrease non-specific toxicity to normal tissue and organs. Here, we propose to use anti- Her2 SUPR peptides to deliver radionuclides to Her2-positive tumors in mouse breast cancer models to establish the efficacy of SUPR peptides as agents for systemic targeted radionuclide therapy.